This invention relates to a device for generating a vacuum, and in particular for an automotive vehicle brake system with brake slip control, including a chamber of variable volume communicating with at least one wheel brake for the purpose of slip-dependent pressure control in the wheel brake cylinders and with the change in volume being effected by means of a vacuum servomotor consisting mainly of the intake manifold of a combustion engine and a carburetor with a throttle valve.
In automotive engineering there exists a wide range of devices the energy source of which is the vacuum in the intake manifold of the combustion engine. A prominent example are the known vacuum brake boosters.
Further, there are known brake slip control devices based on the so-called plunger system. In such systems the wheel brakes communicate with a chamber of variable volume in case of brake slip control. Such a chamber is achieved by means of a piston-and-cylinder arrangement where the piston is retracted out of the cylinder for the purpose of pressure reduction and an increased volume is made available to the pressure medium in the wheel brake. This automatically results in a pressure reduction. For the purpose of renewing pressure build-up, the piston is moved back into the cylinder. It is known in the art to actuate the piston of such a system by means of a vacuum servomotor. The piston then communicates with a movable wall of a servomotor by way of a push rod and the wall divides a housing into a first and a second chamber. Depending on which chamber is communicating with the atmosphere and which chamber has communication with a vacuum source, there will be a pressurization of the wall and, hence, of the piston so as to effect an increase or reduction in the cylinder volume.
Depending on certain external conditions, such as the friction value between the road surface and tires or speed of the vehicle, several control cycles can take place rapidly. In each of these control cycles, the chamber just ventilated must be re-evacuated that is, the vacuum source has to remove a considerable volume air from the servomotor during control.
In such brake systems it is typical to draw the vacuum from the intake manifold of the combustion engine. One shortcoming of such systems is that, in terms of optimum mixture control, the air taken from the servomotor affects the air-fuel mixture control of the engine and in some instances can even lead to the engine dying.